Ceramide selectively inhibits early events in the response of human neutrophils to tumor necrosis factor

Abstract

Cell spreading and the respiratory burst of neutrophils responding to soluble, physiological agents and adherent to model biological surfaces are typically delayed in onset by 15 min or more. The lag period may be a physiologically important feature of the action of such agents on neutrophils in that it may allow for migration before secretion. However, the mechanism that interposes such a long delay between stimulus and response is unknown. Tumor necrosis factor alpha (TNF-alpha) mediates some of its actions by triggering sphingomyelinase to generate ceramide. In adherent human neutrophils, however, exogenous ceramide did not mimic TNF-alpha's ability to stimulate cell spreading, paxillin tyrosine phosphorylation, or the respiratory burst. On the contrary, ceramide suppressed each such response. Ceramide did so by extending the lag period in the cells' response to TNF-alpha. Ceramide extended the lag period whether it was added exogenously or generated endogenously by sphingomyelinase, and the effect was reversible. Remarkably, however, ceramide inhibited cell spreading or the respiratory burst only if added together with TNF-alpha or within the next few minutes. Neutrophils ignored ceramide if it was added later, even if the TNF-alpha-triggered respiratory burst had not yet commenced. These features suggest that an early, brief elevation of ceramide in response to TNF-alpha could mediate the lag period. By temporarily inhibiting tyrosine phosphorylation, cell spreading, and the respiratory burst, ceramide or a functionally similar mediator could serve as a phase coordinator of the neutrophil's response to soluble agonists.